1. Field of the Invention
The invention relates to new coating compositions stable to thermal yellowing, and to their preparation and use.
2. Description of the Prior Art
Polyurethane-polyurea dispersions (PU dispersions) and aqueous formulations of PU dispersions are known state of the art. An overview of the various types and processes for preparing PU dispersions and their aqueous formulations is found for example in Houben-Weyl: “Methoden der Organischen Chemie”, Vol. E 20, pp. 1659-1692 or in “Ullmann's Encyclopaedia of Industrial Chemistry” (1992) Vol. A21, pp. 667-682. By virtue of their combination of positive properties such as mechanical strength, high adhesion to different substrates, solvent resistance, gloss, etc., they find broad use as, for example, paints and coatings. One important field of use of aqueous formulations of ionically modified PU dispersions is in the area of the painting of plastics parts.
The aesthetic and technical requirements mean that plastics parts are usually painted, in order to protect the plastic against external influences, such as sunlight, chemical, thermal and mechanical stress, to achieve particular colours and colour effects, to mask defects in the plastic's surface or to give it a pleasant feel (tactility). In order to improve the tactile properties of plastics parts, use has been made increasingly in recent years of what are called soft feel coating materials.
“Soft for effect” present invention refers to a particular tactual sensation (tactuality) of the coated surface; this tactility can be described using terms such as velvety, soft, rubbery, warm, whereas, for example, the surface of a painted car body or else an unpainted polymer sheet or one coated with a customary clear coat or top coat material and made, for example, of ABS, Makrolon® (polycarbonate, Bayer AG) or plexiglass (polymethyl methacrylate) feels cold and smooth. In tune with the trend of avoiding solvent emissions to the environment, recent years have seen the establishment of aqueous soft feel coating materials based on the polyurethane chemistry, as disclosed, by way of example, in the teaching of DE 44 06 159. As well as an excellent soft feel effect, these coating materials also produce coatings having good resistance and protection for the plastics substrate. It has since been found, however, that these coating materials and coatings often have only an inadequate yellowing stability.
The object of the present invention is therefore to provide coating materials which in addition to the abovementioned mechanical and tactile properties lead, in comparison to prior art coating materials, to coatings with significantly greater stability to thermal yellowing and/or a significantly lower level of thermal yellowing.
As described for example in DE-A 44 06 159, plastics-coating materials having the desired tactile soft feel properties are composed in part of PU dispersions containing no notable amounts of hydroxy-functional groups.
The prior art has disclosed numerous stabilizers and additives which are able to reduce thermal yellowing of binders. In the field of the above aqueous not notably hydroxyl-functional PU dispersion, however, the inhibitory effect of these systems on yellowing is inadequate or they lead to poorer performance properties of the dispersions and coatings, such as poorer stress-strain behaviour or poor compatibilities with other paint or coating components. The known additives are also prone to migration from the coatings produced, so that, over time, unwanted fogging and a tailing off in the yellowing stabilization comes about.
U.S. Pat. No. 5,137,967 describes the preparation of carboxylate-containing PU dispersions which are stable with respect to thermal yellowing and are prepared by the method known as the prepolymer mixing method. For yellowing stabilization, hydrazine is used to chain-extend the prepolymer and dimethylaminoethanol (DMAE) is used as the neutralizing amine for the carboxylic acid groups.
DE-A 32 38 169 describes a process for preparing PU dispersions which uses hydrazine or hydrazides as additives or as chain extenders. Exclusively anionic, carboxylate-functional PU dispersions by the prepolymer mixing method are described.
Hydrazines and hydrazides as chain extenders in polyurethanes are known in principle, for example, from U.S. Pat. No. 4,147,679 or DE-A 23 14 513. In some cases they are also used in mixtures with other chain extenders such as diamines (U.S. Pat. No. 3,415,768). They serve to improve flexibility, hardness, resistance and drying of the coatings.